A star explodes, turns inside-out

Credits: Illustration: NASA/CXC/M.Weiss; Image: NASA/CXC/GSFC/U. Hwang & J. Laming
(PhysOrg.com) -- A new X-ray study of the remains of an exploded star indicates that the supernova that disrupted the massive star may have turned it inside out in the process. Using very long observations of Cassiopeia A (or Cas A), a team of scientists has mapped the distribution of elements in the supernova remnant in unprecedented detail. This information shows where the different layers of the pre-supernova star are located three hundred years after the explosion, and provides insight into the nature of the supernova.

An artist's illustration on the left shows a simplified picture of the inner layers of the star that formed Cas A just before it exploded, with the predominant concentrations of different elements represented by different colors: iron in the core (blue), overlaid by sulfur and silicon (green), then magnesium, neon and oxygen (red). The image from NASA's Chandra X-ray Observatory on the right uses the same color scheme to show the distribution of iron, sulfur and magnesium in the supernova remnant. The data show that the distributions of sulfur and silicon are similar, as are the distributions of magnesium and neon. Oxygen, which according to theoretical models is the most abundant element in the remnant, is difficult to detect because the X-ray emission characteristic of oxygen ions is strongly absorbed by gas in along the line of sight to Cas A, and because almost all the oxygen ions have had all their electrons stripped away.

This Chandra X-ray video shows the elements distribution in supernova remnant Cas A. The distributions of sulfur, silicon, magnesium and neon are similar. Oxygen, which according to theoretical models is the most abundant element in the remnant, is difficult to detect because the X-ray emission characteristic of oxygen ions is strongly absorbed by gas in along the line of sight to Cas A, and because almost all the oxygen ions have had all their electrons stripped away. (NASA/CXC/A. Hobart)

A comparison of the illustration and the Chandra element map shows clearly that most of the iron, which according to theoretical models of the pre-supernova was originally on the inside of the star, is now located near the outer edges of the remnant. Surprisingly, there is no evidence from X-ray (Chandra) or infrared (Spitzer Space Telescope) observations for iron near the center of the remnant, where it was formed. Also, much of the silicon and sulfur, as well as the magnesium, is now found toward the outer edges of the still-expanding debris. The distribution of the elements indicates that a strong instability in the explosion process somehow turned the star inside out.

This latest work, which builds on earlier Chandra observations, represents the most detailed study ever made of X-ray emitting debris in Cas A, or any other supernova remnant resulting from the explosion of a massive star. It is based on a million seconds of Chandra observing time. Tallying up what they see in the Chandra data, astronomers estimate that the total amount of X-ray emitting debris has a mass just over three times that of the Sun. This debris was found to contain about 0.13 times the mass of the Sun in iron, 0.03 in sulfur and only 0.01 in magnesium.

The researchers found clumps of almost pure iron, indicating that this material must have been produced by nuclear reactions near the center of the pre-supernova star, where the neutron star was formed. That such pure iron should exist was anticipated because another signature of this type of nuclear reaction is the formation of the radioactive nucleus titanium-44, or Ti-44. Emission from Ti-44, which is unstable with a half-life of 63 years, has been detected in Cas A with several high-energy observatories including the Compton Gamma Ray Observatory, BeppoSAX, and the International Gamma-Ray Astrophysics Laboratory (INTEGRAL).

These results appeared in the February 20th issue of The Astrophysical Journal in a paper by Una Hwang of Goddard Space Flight Center and Johns Hopkins University, and (John) Martin Laming of the Naval Research Laboratory.

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7 comments

??? So, for this inversion to happen the explosion had to take place in the very core of the star. This doesn't appear to be possible based on what we (think we) know of supernovae behavior. What's going on???

If LaViolette is correct, and new matter is formed most rapidly in regions of highest matter density, the lighter elements would predominate in the core regions of stars. The heavier elements would have higher proportion in regions of the star where the new matter (hydrogen) generation rate is lower, nearer the surface. No need to turn the star inside-out. It was inside-out from the start.

If LaViolette is correct, and new matter is formed most rapidly in regions of highest matter density, the lighter elements would predominate in the core regions of stars

LaViolette is my favorite astronomer, but I don't understand the deduction of yours. During fusion at the core of stars the heavier elements are usually produced. If the fusion inside the core occurs faster, then the highest concentration of these elements should be right there - or not?

If the production of new lighter material in the dense core rapidly outpaces the fusion of those elements into metals, then this core region would be relatively devoid of metals when compared to the outer regions. I suspect that metals can also be produced in the outer or intermediate regions as well. It just becomes a question of relative degree of production rates, versus the new matter nucleation rate.

LaViolette claims that this nucleation rate increases exponentially with matter density, so in such massive supernova candidates, this process can lead to thermal instability and explosive outbursts. (One must consider that genic energy production from photon blue-shifting in the core is also responsible for this instability, according to LaViolette.)

When considering the out-ward flow of material following the super-nova, and considering various 'impediments' to traveling particles, would not the heavier material be less impeded by , eg, dust,etc, and consequently find itself moved to the fringes of the expanding wave-front .. while lighter materials are slowed down in their radial traverse?In this light, the finding seems predictable.But then, im not an astronomer..

LaViolette suggests that iron was formed preferentially in the core due to higher explosive energy located there, propelling the iron outward faster than from the outer layers. He maintains that this supports his starburst model where genic energy production (photon blue-shifting) is much greater in the core where genic energy density is far greater. Core collapse rebound models would - at best- predict all parts of the star expanding outward with equal energy, rather than the core expanding faster than the outer regions, turning the star inside out.

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